1. Field of the Invention
The present invention relates to a process for preparing low-odor hydrogel-forming addition polymers which are based on acrylic acid.
2. Description of the Background
Water-absorbing addition polymers, which are also known as hydrogel-forming addition polymers or as superabsorbent polymers (hereinafter abbreviated to SAPs), are capable of absorbing and hence binding aqueous fluids by forming a hydrogel. SAPs therefore find use in hygiene articles such as diapers, incontinence inserts and briefs, sanitary napkins and the like to absorb body fluids. A comprehensive overview of SAPs, their application and their production is given by F. L. Buchholz and A. T. Graham (editors) in Modern Superabsorbent Polymer Technology, Wiley-VCH, New York, 1998.
Among SAPs, those based on acrylic acid constitute a particularly important class of materials. However, their process of preparation is such that they generally contain a large amount of volatiles or elutables, e.g. unconverted monomers (residual monomers) and especially unconverted acrylic acid monomer. Yet, SAPs to be used in hygiene articles or else in food packaging materials or as assistants in the agricultural sector shall in principle have low levels of volatile and elutable materials. A reduction in these levels is also desirable from an ecological viewpoint.
There have been various proposals for reducing the level of volatile residual monomer in acrylic acid based SAP. An overview may be found in EP 372706 for example. Proposals include the irradiation of SAP with ultraviolet light (JP 62260906), the addition of amines (JP-A 5040649) or sulfite or hydrogen sulfite (U.S. Pat. No. 4,306,955), the extraction with hydrophilic organic solvents or with supercritical CO2, the use of specific initiator combinations, such as redox initiators combined with azo initiators, or the use of microorganisms (U.S. Pat. No. 4,742,114).
EP-A 372706 discloses preparing acrylic acid polymers having a low residual monomer content by using an aqueous acrylic acid solution obtained by first admixing an acrylic acid solution with a molar excess of a base and, following a delay time, adding further acrylic acid to set a degree of neutralization in the range from 20 to 100%.
EP-A 574260 discloses a similar procedure, except that the acrylic acid used contains less than 1000 ppm of β-hydroxypropionic acid. The acrylic acid is always freshly distilled for this purpose.
Existing methods are in some instances effective in reducing the measurable residual monomer content of SAPs based on acrylic acid. Yet, these SAPs frequently have an unpleasant odor. True, this unpleasant odor does not in principle diminish their water-absorbing properties, but it leads to a reduced customer acceptance in hygiene article applications in particular.